Neivamyrmex bohlsi. Image by: Alexander Wild (www.alexanderwild.com)
Ever watched ants forage for food? No? Do not fret then, for I have already done it for you. One afternoon on a partly cloudy day, I went to a local park and observed some rather small, vicious-looking red ants (order Hymenoptera, family Formicidae) as they traveled on the concrete of the sidewalk. Nearby was a field of grass, where their home probably was. What interested me about these ants was the way they traveled. There were two apparent intermixing groups of the same ants traveling in opposite directions, which can be seen in the rough diagram below.
About half of the ants were heading away from the grass while half were heading into the grass. I noticed the ones heading toward the grass carrying some small white crumb-looking objects and decided to see where they were coming from exactly, but I found nothing on the other end as it appeared they had already scavenged all of the food. The ants at the head of the line were walking in strange zigzags while staying close. The two main eye-catching observations of these ants then were that despite the fact that there were ants traveling in opposite directions, they traveled near each other and that the leaders of the line seemed to follow nonlinear paths while searching for food or necessary materials.
Previous research done on zigzag movements of animals determined that these animals may be gathering “parallax information,” or figuring out where they are in relation to their goal (Lent, Graham, and Collett, 2013). Other research suggests that ants possess powerful spatial memory and even use landmarks to determine where they are (Wehner, Michel, and Antonsen, 1996). Thus, moving in strange paths instead of walking straight would allow ants to survey their surroundings more effectively and perhaps more efficiently travel to and from the colony home. By having large numbers of ants forage together, their “collective” memory and information-gathering increases greatly, providing additional benefits for the colony in the form of food and other resources.
As for why the ants seemed to walk along the same path to and from the colony home, I suspect that it had to do with the fact that ants are drawn to local concentrations of pheromones, which are chemical signals that affect members of the same species (Perna et al., 2012). Because there were many ants that had traveled to the same location, the density of pheromones would have been highest near the line traveled, and thus, it only makes sense for these ants to travel back along the same way.
How might have these traits arisen? Ants, along with most or all other members of the order Hymenoptera, have their sex determined by a haplodiploidy system in which the number of chromosomes they possess determines their sex. This results in an interesting phenomenon where sisters are more closely related than daughters are to their mothers and is, according to a theory, the driving force behind altruistic colonies where members give up reproduction to ensure the success of the colony (Trivers and Hare, 1976). Thus, the colonial ants who can best work together are the ones who pass on the most genes by being the most successful colony, supporting each other and the reproductive queen. Ant species would be expected to express any cooperative behavior that arose because it would aid in their survival, and an example of this is pheromone-based grouping. By grouping together by cue of pheromones, they can combine information gathered from zigzagging, and ants who were “confident” in their discovery could lead the charge while others follow a pheromone trail, allowing groups of ants to forage efficiently.
Cooperative behaviors allow us to ask questions such as, “What is the basic unit of the organism in the case of an ant?” It is indeed as if they are all parts of a working machine. However, we still consider each ant as an individual, and its rather simple behavior of following its comrades leads to a larger scale movement of ant trails. The visual zigzagging allows the ants to gauge their position while searching for food in multiple directions while the pheromones keep them together. If the pheromone-sensing should fail, they still have a failsafe in their recognition of visual landmarks. Note that we would have expected zigzagging to only involve in colonial ants as zigzagging in solitary ants would have likely made it very dangerous for a lone individual by increasing travel time, and they would not have likely had a center they called their home. There is also the possibility though that pheromone grouping is what led to the formation of colonies, but we can assume that pheromone grouping did not help solitary ant ancestors as they would have been drawn together to compete for resources, reducing their success.
With pheromone-sensing and zigzagging in tandem, ants can efficiently locate food as a group (by zigzagging), stay together to collect food in case there is too much or in case it is too large (by pheromone trails), and find their way back (by parallax information obtained from zigzagging and by pheromone trails). Such efficiency would result in the ant colonies outcompeting others and surviving through time. This outcompeting would have also resulted in proliferation of genes involved in following pheromone trails and gathering parallax information by natural selection. Thus it is logical to conclude that these behaviors arose with colonial animals, were proliferated by their effectiveness in helping colonies thrive, and are seen still today because natural selection decided they were the most effective way to survive for these ants.
References
Lent, D.D. , P. Graham, and T.S. Collett. (2013) Phase-Dependent Visual Control of Zigzag Paths of Navigating Wood Ants. Curr. Biol. 23: 2393-2399.
Perna, A., B. Granovskiy, S. Garnier, S.C. Nicolis, M. Labedan, G. Theraulaz, V. Fourcassie, and D.J. Sumpter. (2012) Individual rules for Trail Pattern Formation in Argentine Ants (Linepithema humile). PLoS Comput. Biol. 8: e1002592.
Trivers, R.L., and H. Hare. (1976) Haploidploidy and the evolution of the social insect. Science 191: 249-263.
Wehner, R., B. Michel, and P. Antonsen. (1996) Visual navigation in insects: Coupling of egocentric and geocentric information. J. Exp. Biol. 199: 129-140.
A. Wild, An underground trail of a South American army ant (Neivamyrmex bohlsi), <http://www.alexanderwild.com/keyword/ant%20trail/i-s6ptZFs/A>.
Tags: ants, foraging, group behavior